The present invention relates to a feed device for an electronically controlled pattern sewing machine and, more specifically, to a feed device which is driven directly by a stepping motor.
In the conventional electronically controlled pattern sewing machine, in general, the needle bar reciprocating mechanism and the vertical feed dog driving mechanism are driven by the main motor of the sewing machine, while the needle bar swinging mechanism and the horizontal feed dog driving mechanism are driven by stepping motors, respectively.
Recently, proposed a pattern sewing machine having a simple driving system including a horizontal feed dog driving device which is driven individually and directly by a stepping motor, and capable of accurately controlling the feed operation on the basis of the latest required feed stroke.
By way of example, U.S. Pat. No. 4,286,532 discloses a sewing machine equipped with a feed device which is driven individually. In this sewing machine, the phase of the main shaft of the sewing machine is detected by a main shaft phase detector, the feed shaft is driven in synchronism with the main shaft, and the phase of the feed shaft is regulated on the basis of the main shaft phase detection signal provided by the main shaft phase detector so that the phase of the feed shaft coincides with a predetermined phase to stop the feed motion of the feed shaft at a fixed timing.
Driving the feed device of an electronically controlled pattern sewing machine in synchronism with the reciprocating needle bar by an individual stepping motor or the like, in general, has the following problems adversely affecting the sewing quality.
Since the rotating speed of the main motor for driving the needle bar, hence, the stitching speed, is variable over a wide range according to the stitching condition, the feed stroke for each stitch is variable, namely, the number of driving pulses given to the stepping motor is variable, and the period of the driving pulses is constant, the feed stop timing varies when the feed start timing is fixed; consequently, the feed motion is stopped before the needle thread is tightened by the cooperative effect of the takeup motion of the thread take-up lever and the feed motion, and threby the needle thread is held between the work and the throat plate.
On the other hand, the feed device disclosed in U.S. Pat. No. 4,286,532 is capable of properly controlling the feed stop timing. However, the same feed device needs a main shaft phase detector for detecting the phase of the main shaft, and electrical signaling means for regulating the phase of the feed shaft so that the phase of the feed shaft corresponds to that of the main shaft, which complicates the constitution of the device and increases the manufacturing cost.